Prior indoor chairs could be dining chairs, office chairs and lounge chairs. They generally use woods, metals and plastic as manufacturing materials under the consideration of structures and shapes in design. Using various materials will present the different qualities and corresponding structure techniques of the chairs. The application of materials generally depends on the consideration of design. Due to the limitation of physical properties and strengths of different materials, Bendability of woods is not well and the strength thereof is worse than the metal. Metal has a higher strength, but also the bendability thereof is limited. Plastic material has high plasticity to match design demands and is suitable to form various 3D-streamline art models.
Meanwhile, using plastic would lower consumption of wood and is reusable and thus it is economic. Due plastics is quite and the strength thereof is weak, synthetic material is applied to promote the combined strength and durability of the plastic parts. Using downwards expanding legs in chairs usually increase coverable and stability of chairs and also crossing linkages between legs are used to reinforce the strengths of legs and enhance ability of supporting of goods thereon. However, it causes un-removable members between legs and limits the advantages of plastics for easily molding and matching the model design idea in art shape.
Moreover, volumes of plastic chairs are constant so that costs for transportation are increased. Some synthetic material assemblies are used to be combined with the chairs. Although it has shapeable and is full of artistic performance, the combination of metal parts and chair legs often explode due to crushing of plastics, as a result lifetimes of the chairs are reduced. In particular, threaded parts between bottoms of metal chairs and the plastic legs often crush to cause loosen chair legs fail in use. Replacing new leg is one permissible solution, however. To abandon the whole leg with crushing thread only causes waste of goods. Therefore, there is a new task in technique invention to improve the connection between the metal chair bottom and plastic leg for prolonging lifetime and increasing the worth thereof, minimizing replacement costs.